1. Field of the Invention
This invention relates generally to the field of cable failure detection, and more particularly, to an apparatus for detecting excessive chafing of a cable arrangement against an electrically grounded structure prior to actual cable failure.
2. Background Information
It is well known that chafing between a pair of objects occurs when the pair of objects are brought into contact with each other and relative motion between the objects takes place. It is further well recognized that continued chafing between the pair of objects will, over time, result in erosion-induced damage to one or both of the objects. Consider, for example, the chafing which occurs when an electrical cable which forms a portion of an electrical system contacts the system hardware or support structure at some point along the cable's length and relative motion, i.e., chafing, between the cable and structure occurs. This relative motion can result from vibration of or within the electrical system, or from the frequent removal and installation of a replaceable unit to which the cable is attached. Since the electrical cable which is experiencing chafing is normally embedded in the electrical system and thus hidden from view, chafing goes largely undetected. If the structural member against which the electrical cable is chafing is electrically grounded, the cable itself will fail when the protective insulation surrounding the cable's current-carrying conductor erodes sufficiently as a result of chafing to allow the conductor to contact the grounded structural member. Grounding the current-carrying conductor not only results in an interruption of current flow to the load device connected with the cable, but may also result in damage to the source which supplies power to the load device via the electrical cable.
As an example of the serious problems which may arise as a result of undetected cable chafing, consider the electrical systems of present day aircraft. These electrical systems predominantly utilize unarmored cables in order to reduce the aircraft's overall weight. It is readily apparent that the failure of an aircraft electrical cable during flight as a result of chafing between the electrical cable and the grounded air frame can cause engine failure or the loss of navigational instrumentation. Obviously, these situations would greatly endanger the safety of all on-board personnel.
Many of the complex equipment systems presently utilized in industries related to transportation and manufacturing include electrical and hydraulic/pneumatic subsystems; and it is apparent that portions of the electrical cabling and hydraulic and pneumatic tubing utilized in these subsystems will invariably be subjected to chafing during their operating life. Chafing-induced cable or tubing failures that result in shutdown of the equipment system will require valuable time to determine the cause of the shutdown, locate the failure and effect a repair. This situation is complicated by the fact that, at present, there is no easy way to determine the exact location of a chafing-induced failure even after it is determined that the system shutdown actually resulted from cable or tubing chafing. In addition, in the case of chafing-induced electrical cable failures, the exact location of the failure may be detected utilizing presently available test or detection equipment only if contact is maintained between the current-carrying conductor of the chafed cable and the grounded structural member or equipment support hardware. If the current-carrying conductor of the chafed cable is only momentarily brought into contact with the grounded structural member or system support hardware (as in the case of transient chafing due to vibration), conventional failure detection equipment is incapable of determining the exact location of the cable failure.
Therefore, there is a need for an apparatus which includes a sacrificial covering positioned in surrounding relation with an electrical cable or other tubing which is operable in conjunction with a detector circuit connected therewith to detect chafing of the sacrificial covering against an electrically grounded structural member. Since the sacrificial covering surrounds the cable or other tubing, any chafing-induced erosion experienced by the cable/sacrificial covering arrangement will take place in the covering rather than in the cable. The detector circuit should be operable to provide an indication of chafing-induced erosion of the sacrificial covering well before the cable itself experiences actual chafing against the grounded structural member. This timely indication will allow the chafing condition to be corrected prior to chafing-induced failure of the cable or tubing itself. In addition to providing an indication of chafing-induced erosion of the sacrificial covering, the detector circuit should be capable of providing an indication of the distance to the chafing location measured from a reference end of the sacrificial covering in order to avoid the need for a visual inspection of the entire covering to locate the chafe. Finally, the detector circuit should be capable of providing an indication of chafing-induced erosion of the sacrificial covering in the event that the chafing is transient or intermittent.